1. Field of the Invention
The present invention relates to an optical glucose sensor chip and a method of manufacturing the optical glucose sensor chip.
2. Description of the Related Art
As an optical glucose sensor chip, for example, a less invasive type blood sugar level measuring chip has been developed which indirectly measures a blood sugar level by extracting body fluid of subcutaneous tissues. This sensor chip has a structure including a glass substrate, a pair of gratings which are formed on a surface of the substrate and introduce light to or emit the light from the substrate, and a glucose sensing membrane which is positioned between the gratings and formed on the surface of the substrate. This glucose sensing membrane contains a color reagent substrate (for example, 3,3′5,5′-tetramethylbenzidine [TMBZ]), a first enzyme (for example, glucose oxidase [GOD]) that oxidizes or reduces glucose, a second enzyme (for example, peroxidase [POD]) generating a material that reacts with a product obtained by the oxidation or reduction of glucose to make the color reagent substrate exhibit color, and a film forming high-molecular compound (for example, a cellulose derivative such as carboxymethyl cellulose [CMC]).
When a sheet gel is disposed between the skin and the above sensing membrane to apply an electric field in a glucose sensor chip having such a structure, glucose in a subcutaneous tissue solution penetrates the gel from the skin and reaches the above sensing membrane. At this time, the above TMBZ as a color reagent substrate in the sensing membrane makes to exhibit color due to the reaction between glucose and GOD or POD. When light is made to be incident to the above substrate and to be refracted on the surface of the substrate and on one of the above gratings, the light propagates through an interface between the above substrate and the sensing membrane containing color-formed TMBZ, is refracted on the interface between the substrate and the other grating and is received by, for example, a light receiving element. The intensity of the received laser light is less than the intensity (initial intensity) of the laser light received by the light receiving element when the above sensing membrane has not exhibited color. For this reason, the concentration of the above glucose can be detected from the ratio of reduction of the above light intensity.
However, when the above sensing membrane is stored and used for a long time, the activity of the first and second enzymes in the membrane rapidly deteriorates. Examples of the cause of the deterioration include a variation in the pH of the sensing membrane, a variation in the ionic strengths of the first and second enzymes and hydrolysis of the first and second enzymes. When the first and second enzymes deteriorate, the first enzyme reacts insufficiently with glucose which is the subject to be measured. The reduction in the reactivity with glucose reduces the generation of the material that aids the color-forming material obtained by the subsequent reaction with the second enzyme in exhibiting color, with the result that this causes a reduction in the reactivity with the color-forming material and a reduction in the degree of the color to be exhibited, bringing about a deterioration in the sensitivity of the glucose sensor chip.